1. Field of the Invention
This invention relates to a wire electric discharge machine for processing a workpiece (an object to be processed) by repetitively inducing pulse-shaped discharge across a wire electrode and the workpiece.
2. Description of Related Art
In this type of wire electric discharge machine, it is known that when the interval of the discharge pulses to be applied across a wire electrode and a workpiece, that is, a discharge period, is set to an excessively small value, the discharge is concentrically induced and breakage of the wire electrode is liable to occur. On the other hand, when the discharge period is set to an excessively large value, the processing speed is reduced.
It is also known, in this type of wire electric discharge machine, that as the thickness of the workpiece is increased, the confronting area between the workpiece and the wire electrode is increased. Conversely, as the thickness of the workpiece is decreased, the confronting area between the workpiece and the wire electrode is decreased. Therefore, the concentrated discharge, which causes the breaking of the wire electrode, occurs less often as the thickness of the workpiece is increased.
Accordingly, the conventional wire electric discharge machine has been so designed that the user sets the most suitable discharge period in accordance with the material and the thickness of the workpiece before processing the workpiece, and the processing of the workpiece is carried out using the set discharge period.
There are some cases where a workpiece has a varied thickness, such as a workpiece having a stepped portion that must be processed using the wire electric discharge machine. In order to carry out the processing of such a workpiece with an optimum discharge period at all times, the discharge period must be altered at a timing based on the thickness of the workpiece.
However, the wire electric discharge machine is frequently driven using an unmanned operation system because the processing time for the workpiece is long and thus it is difficult to alter the discharge period at the time the variation in thickness of the workpiece occurs.
Therefore, in a case where a workpiece having stepped portions is processed using the conventional wire electric discharge machine, in order to prevent breakage of the wire electrode, the discharge period must be set to a discharge period which is suitable for the thickness of the thinnest portion of the workpiece. Therefore, there occurs a problem that the processing speed at thick portions of the workpiece is reduced to a value lower than the optimum processing speed resulting in a longer processing time.